Conventionally, belt transmission systems are employed for driving auxiliary systems such as a car compressor and an oil pump of an automobile.
Such a belt transmission system is adapted to transmit a driving force from a crank shaft of an engine to an auxiliary system through pulleys and a belt at a predetermined transmission ratio. Therefore, the rotational speed of the auxiliary system is increased as the rotational speed of the crank shaft is increased. The operational efficiency of the auxiliary system is generally increased with the increase in the rotational speed thereof, but the efficiency tends to decrease when the rotational speed exceeds a predetermined level.
Therefore, driving the auxiliary system at a rotational speed higher than a required level leads to wasteful energy consumption, and adversely effects the durability of the auxiliary system. In view of this, there has been proposed a belt transmission system which is capable of adjusting the rotation speed of an auxiliary system.
Such a belt transmission system is disclosed, for example, in Japanese Unexamined Patent Publication No. 2-500261 (W). The belt transmission system according to this publication employs a variable diameter pulley which is capable of varying the contact diameter of a belt stretched therearound.
The variable diameter pulley includes a multiplicity of belt engagement bars arranged in a circular pattern around a rotation shaft thereof and resiliently and radially outwardly biased by biasing means. The diameter of the circular pattern is equivalent to the effective diameter of the variable diameter pulley. The multiplicity of belt engagement bars are radially inwardly moved in unison in resistance to a biasing force applied by the biasing means to vary the effective radius of the variable diameter pulley (the contact radius of the belt).
More specifically, the variable diameter pulley has a pair of rotation plates opposed to each other and respectively formed with a multiplicity of grooves extending radially and spirally in opposite directions. Opposite ends of the respective belt engagement bars are supported by corresponding grooves of the rotation plates. Thus, the effective diameter can be varied with the respective belt engagement bars kept in the circular pattern as the rotation plates are relatively rotated. Used as the biasing means is a twist coil spring which is disposed between the rotation plates and adapted to rotatively bias the rotation plates toward each other in such directions that the effective diameter is increased.
The variable diameter pulley according to the aforesaid publication has a large number of components because of the provision of the multiplicity of belt engagement bars. In addition, these belt engagement bars should be kept in the circular pattern for varying the diameter of the circular pattern. Therefore, the construction of the variable diameter pulley is complicated.
Further, when the multiplicity of belt engagement bars are moved to vary the diameter of the circular pattern, frictional resistance occurs between the opposite ends of the respective belt engagement bars and the corresponding radial grooves. Since the multiplicity of belt engagement bars each have two friction points, the variable diameter pulley has a great number of friction points. As a result, the variable diameter pulley suffers a great frictional resistance, making it difficult to smoothly change the rotation speed.
The diameter of the circular pattern depends on radial positions of the belt engagement bars at which a force exerted by the belt for radially inwardly biasing the belt engagement bars is balanced with a biasing force of the twist coil spring as the biasing means. Therefore, where the biasing force is small, the effective diameter of the variable diameter pulley may fluctuate in sensitive response to a torque fluctuation. The fluctuation of the effective diameter of the variable diameter pulley could be prevented by increasing the biasing force. However, this may lead to an increase in frictional torque, resulting in a less smooth change of the rotational speed and a greater loss of transmission torque.
It is an object of the present invention to provide a variable diameter pulley which features a simple construction and smooth change of the rotational speed, and is less sensitive to the torque fluctuation.